Drawing hair is something that most budding artists struggle with. Often times, the frustration comes from not knowing how to break free from using the blending stub or not having a handle on how to use deep shading effectively. This text should shed some light on the subject of drawing and shading realistic hair. This is based on personal experience and stuff I've read online, so it's not too technical to understand.
Most of the portraits I've seen by new artists, including myself when I started drawing faces, have hair that is drawn with cartoon-like detail. Instead of shading it in, a few strands are drawn in to give it the effect of being hair. While this is fine for sketches and doodles, there comes a time when it's just not sufficient for the subject.
The first thing to remember when shading is that it's a lot easier to darken a spot later than to go back and try to undo shading. Therefore, it's best to make the first layer of shading light, then go back over it a second time. The best pencil for the first layer is probably a normal HB.
Draw each strand, instead of trying to shade a large area at once. The process gives the hair striking detail and the result looks very impressive. Make sure that all the strokes go the same direction. Lift the pencil between each strand, instead of going back and forth. That makes the shading more even and eliminates loops at the end of the hair. Strokes can go up or down but should start dark and get progressively lighter.
One element of the hair that can make or break the entire picture is white space. There should not be white space between strands, since those areas would not have light hitting them so directly. The white areas should be filled in with more strands. White space can be used very effectively for reflections. The way the hair shines can give life to otherwise plain looking hair and change the entire tone of a picture. The easiest way to make a white spot is at the peak of a wave. Just make your strands end on either side of it without meeting in the middle. The white space should be wide on the first layer, since the exact shape and size can be decided on the second layer of shading.
After all the hair is in place and you are satisfied with the way it looks, it's time to do the second layer. Go over all the shading, deepening the tones and giving more contrast to the grooves and dips. It may be good to use a sharp 3B pencil on the black parts. Remember that lighter colored hair has more focus on contrast between shadows and light but less value in the flat areas.
One detail that can make a picture look interesting is loose strands of hair. This is something that can be difficult to be realistic with sometimes. It's easy to get carried away and, before you know it, the picture looks cluttered. Remember that hair is thin and light; so don't use bold strokes to make loose strands.
When drawing a complete portrait, the hair can be used to control the focus on the detail of the face. Perfect hair will make up for a less than perfect face because the hair demands attention. Likewise, rough hair will make the face stand out. If you're having a hard time with the face, spend a few hours making the hair perfect. If you nail the face, consider making more vague hair with a softer pencil.